The invention is directed to a system for the thermal treatment of meal-like raw materials, particularly in the manufacture of cement clinker from raw meal, whereby the raw meal is thermally treated in a burning process by pre-heating, calcining, sintering and cooling, and the exhaust gas stream of the sintering unit (rotary kiln) and an exhaust air stream (tertiary air) of the cooling unit (clinker cooler) are used in such a way for calcination of the raw meal in the calcining unit that at least one burning location is present both in the exhaust gas channel of the rotary kiln as well as in the tertiary air channel, whereby the gas/raw meal/fuel suspension coming from both channels is redirected in the calcining unit and is introduced into the lowest cyclone of the suspension-type cyclone pre-heater system for the purpose of separating the calcined raw meal from the gas stream.
Systems for the manufacture of cement clinker from raw meal of the type set forth above having a calcining stage that precedes the rotary tubular kiln and that is equipped with secondary firings must be in the position of producing a raw meal calcined to a great extent before introduction into the rotary tubular kiln. At the same time, the builders and operators of cement clinker production lines are confronted with stricter and stricter demands for low emission values of pollutants such as NO.sub.x and CO.
For reducing such noxious emissions, it is known (for example, EP-B-0 222 044 as well as EP-B-0 526 770) in cement clinker production systems of the type initially cited to burn the fuel in the ascending rotary kiln exhaust gas conduit sub-stoichiometrically, i.e. starved for oxygen, for the purpose of creating a CO-containing reduction zone for reduction of the pollutant NO.sub.x that, in particular, has been formed due to the high-temperature burning in the rotary tubular kiln (thermic NO.sub.x). In the neighboring channel, what is referred to as the tertiary air channel and through which a portion of the hot clinker cooler exhaust is conducted, fuel is burned over-stoichiometrically, i.e. with an excess of oxygen. The heat released in the fuel combustion is used for calcination of the raw meal in any case. As viewed in a downstream direction of the suspension flow, the CO not consumed in the NO.sub.x reduction zone is burned with excess oxygen from the tertiary air channel after being merged with the rotary kiln exhaust gas channel, whereby the residual burning is additionally promoted by the flow redirection of the suspension in the calcining unit.
Whereas the firing in the rotary tubular kiln is a high-temperature combustion with long burner flame, whereby mainly thermic NO.sub.x enters into the exhaust, the temperature in the usually flameless burning in the calcination unit does not exceed the calcination temperature of approximately 850.degree. C., whereby it is mainly fuel NO.sub.x that arises. This burning temperature/calcination temperature, however, is too low for burning inert fuels that are difficult to ignite or, respectively, difficult to burn (what are referred to as secondary fuels) such as, for example, inferior coal, etc., so that such substances have been hitherto burned in the rotary tubular kiln itself or were not capable of being utilized in the production of cement clinker.
It is also known (trade periodical "World Cement", October 1998, pages 83-88) to burn inert secondary fuels outside the rotary tubular kiln, namely in a vertically arranged, cylindrical calcining reactor, through which the suspension flows from bottom to top. In this modification, wherein the raw meal is introduced in two separate height levels and the fuel is introduced into the calcining reactor at the bottom, this functions in the fashion of a fluidized bed reactor; for dependable operation thereof given utilization of a large-sized secondary fuel, it would be necessary to grind the secondary fuel. Specific measures are not implemented for achieving low NO.sub.x emissions. In another known modification, a vertically arranged, cylindrical calcining combustion chamber has the tertiary air/raw meal/fuel suspension flowing through it from top to bottom. Inert fuels are thereby employed. A central flame, which extends from top to bottom in the combustion chamber, heats the entire reaction space such (for example, 1000.degree. C.) that the substances that are difficult to burn ignite better. Apart therefrom that the inert fuels--particularly when they are not ground--drop rapidly from top to bottom through the combustion chamber with short dwell time, so that they do not have enough time to completely burn out, this known calcination system also does not address specific measures for achieving an optimally low-pollution burning of secondary fuels outside the rotary tubular kiln given reduced emissions of pollutants such as, in particular, NO.sub.x and CO.